Gombe has been bombed before but has never had the fighters attempt to take it over.

Witness Abdul Hassan said soldiers ran away after the checkpoint was overwhelmed. The fighters then burned down a police station on the outskirts of the town, he said.

"I crossed a river and ran into the hills," he said. "I'm still there and I can hear the fighting."

Others were staying indoors. Witness Hussaina Maji said she was unable to leave her house for fear of being caught in cross fire.

A Nigerian fighter jet encircled the city but made no attempt to attack the fighters, another witness named Kabiru Na-Gwandu said.

He said the residents had been warned to evacuate Gombe.

Gravest threat

The military did not immediately respond to a request for comment.

Boko Haram's struggle for an Islamic state has become the gravest security threat to Africa's biggest economy and top oil producer, killing thousands, kidnapping hundreds and increasingly threatening neighbours Cameroon, Chad and Niger - all of which are now also battling the fighters.

Nigeria has postponed a presidential election that had been due on Saturday, for six weeks, citing the security threat from the armed group.

In May 2013, the government declared a state of emergency in Adamawa, Borno and Yobe provinces, but that has failed to curb the deadly violence.